The present invention relates to electric motors and, in particular, to permanent magnet synchronous machines (PMSM) and, more particularly, to a method and apparatus for automatically identifying electrical parameters in sensor-less PMSMs.
Permanent magnet synchronous machines are electric motors having a rotor holding a permanent magnet that may turn about an axis within a stator. The stator holds conductive coils that may be energized to create a rotating magnetic field. The rotating magnetic field is coordinated with the rotor position to draw the rotor along synchronously.
Simple PMSMs employ power transistors to switch the electrical currents in the stator coils to create the necessary rotating magnetic field. “Sensor-less” PMSMs eliminate the rotor position sensors and deduce rotor position from its effect on the electrical signals used to drive the stator coils.
Most motor drives for sensor-less PMSMs deduce rotor position using one of two strategies: (1) signal injection methods injecting a high frequency signal into the stator; and (2) model-based methods based on stator terminal voltages and currents.
In signal injection methods, the rotor position/speed is estimated using a high frequency voltage or current carrier signal superimposed on the fundamental excitation of the stator to track the rotor position. The signal injection method does not need machine parameters; however, it has limitations caused by the extra losses due to the injected high frequency carriers. Therefore, the signal injection sensor-less method should only be used in low speed ranges including zero speed.
In model-based methods, the rotor position/speed is estimated from the stator voltages and currents based on the fundamental component of the back electromotive force (EMF) or flux linkage. Consequently, most model-based methods fail at low and zero speeds because back-EMF is speed dependent. Further, most model-based methods need motor parameters such as q- and d-axis self-inductance and flux linkage to operate effectively.
Motor parameters are not only needed for model-based methods but are also important in maintaining: (1) high performance, maximum torque per ampere (MTPA) control in the constant torque region, and (2) high performance, flux weakening control. Saturation effects in the motor parameter of self-inductance (e.g., q-axis self-inductance) are important.
Ideally motor parameters could be collected automatically during initial commissioning. Current methods to collect these parameters, unfortunately, either are not suitable for sensor-less motors, require knowledge of other machine parameters and are thus not comprehensive, or provide for incomplete collection of the necessary parameters, treating self-inductances as constants.